Quantum-dot displays could outshine their rivals

The quantum-dot LEDs require just 3 to 4 volts to run for over 300 hours non-stop

(Image: Nature)

The size of the quantum dots can be tuned to produce several different colours

(Image: Nature)

The brightest quantum-dot LEDs yet made could provide lighting for displays that are clearer and richer in colour, as well as being cheaper to make, than existing ones.

The devices could be used to make better displays for mobile phones and PDAs, and to light larger flat-panel TV screens, say researchers based in China and the US.

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Quantum dots are nanoscale semiconductors that confine electrons in three dimensions. In this case, the quantum dots have a cadmium selenide core and a zinc sulphide “shell”. Electrons are excited to higher energy levels in the core and the shell, then fall into the empty spaces, or “holes”, left behind. The dot then forms an “exciton” and emits a particle of light.

Changing the size of a QDLED makes it emit a different wavelength of light – producing red, orange, yellow, or green light. The devices also only need about 3 to 4 volts to operate and can run for over 300 hours without losing any brightness.

Although standard LEDs are far more efficient, QDLEDs could be better in other ways. The range of colours and intensity of light produced by QDLEDs promise to be better than alternative technologies.

“The brightness of the best LCD monitor on the market today is 500 candelas per square metre and the brightness of room light is about 2000 cd/m2,” says Andrew Wang of Ocean NanoTech in Fayetteville, Arkansas, which developed the quantum dots. “Our QDLEDs have reached 9000 cd/m2 in brightness, which makes them the brightest in the world.”

Purer colours

QDLEDs are also relatively easy to make, using solution-processing techniques, such as spin coating, ink-jet printing and roll-to-roll printing, which is useful for flexible-screen applications. Organic LEDs, by contrast, are made using more complicated techniques like vacuum evaporation or vapour deposition.

Although similar devices were made by researchers at MIT in 2002, their QDLEDs had brightness values of around 7000 cd/m2 and contained only a single layer of quantum dots. The new devices contain multiple quantum dot layers to make them brighter, Wang explains.

The Ocean NanoTech researchers, together with Yongfang Li and Qingjiang Sun at the Chinese Academy of Sciences, are now working hard to optimise the structures of the quantum dots to further increase the lifetime and power efficiency of the devices.

For the technology to be incorporated into a product, however, it may be necessary to demonstrate the same physical phenomena using a material other than cadmium, which is a highly toxic heavy metal.